Polyolefins such as propylene polymers or propylene-α-olefin copolymers are inexpensive and yet excellent in mechanical properties, heat resistance, chemical resistance, water resistance, etc., and thus, they are used in various fields. However, such polyolefins have no polar groups in their molecules and thus are usually of low polarity and difficult to bond or apply for coating, and their improvements have been desired. Accordingly, various treatments have been tried such as chemical treatment of the surface of a polyolefin molded product with a reagent or the like, or oxidation treatment of the surface of a molded product by such a means as corona discharge treatment, plasma treatment or flame treatment. However, such methods not only require special apparatus but also the effects for improving the coating property or the adhesive property have not been necessarily adequate.
Therefore, in an attempt to impart a good coating property or adhesive property to a polyolefin such as a propylene polymer by a relatively simple method, a so-called chlorinated polypropylene or an acid-modified propylene-α-olefin copolymer, and further an acid-modified chlorinated polypropylene have been developed. Such a modified polyolefin is applied to the surface of a polyolefin molded product as a surface treating agent, an adhesive or a coating material. The modified polyolefin is usually applied in the form of a solution in an organic solvent or a dispersion in water. With a view to safety and health, and reduction of environment pollution, an aqueous dispersion is usually preferably employed.
For example, an aqueous resin dispersion having an acid-modified chlorinated polypropylene hydrophilically modified by using a surfactant and a basic substance (JP-A-3-182534) or an aqueous resin dispersion having an acid-modified polyolefin hydrophilically modified by using a surfactant and a basic substance (JP-A-6-256592 (U.S. Pat. No. 5,534,577), JP-A-2004-002842 (US2005-124753)) may be mentioned. However, such a method has had a problem that in order to make dispersed particle sizes to be fine, it is required to add a large amount of a surfactant, and consequently, a coating material employing such an aqueous dispersion is poor in water resistance or chemical resistance. Further, after coating, the surfactant may sometimes bleed out on the coated surface, thus leading to a defective appearance. Thus, a further improvement has been desired.
Further, there is an aqueous resin dispersion having dispersed in water a polymer having functional segments block-copolymerized to polyolefin segments (JP-A-2001-288372 (US2003-055179). However, the dispersed particle sizes can not be said to be sufficiently fine, and it is not possible to form the dispersion without using a surfactant at all, and a further improvement has been desired.
Further, in a case where the above-mentioned modified polyolefin is to be used as a water base paint or ink, in order to satisfy the performance other than the adhesion, it is common to incorporate a binder resin such as an acrylic resin, a polyurethane resin, a polyester resin, a vinyl acetate resin or an epoxy resin, or a pigment such as titanium oxide or carbon black, as the case requires. However, such components usually do not have high adhesion to a propylene polymer substrate, and accordingly, there is a problem such that even if the propylene polymer resin itself may have a sufficient substrate adhesion, after mixing, the substrate adhesion substantially decreases.
Therefore, there is a case of a water base paint wherein an ethylene polymer resin having a small particle size is incorporated to a binder resin such as an acrylic resin (JP-A-2004-083787). However, as compared with such an ethylene polymer, with a propylene polymer, emulsification is further difficult, and with respect to a water base paint containing a propylene polymer, it has been strongly desired to solve the above-mentioned problem.
On the other hand, a polyolefin has a low polarity as mentioned above and is thus difficult to emulsify. It has been attempted to emulsify it by using a large amount of a surfactant, or to make the particle size to be fine by using a special emulsifier. For example, an emulsion having a modified polyolefin dissolved in e.g. an aromatic solvent and hydrophilically modified by using a basic substance or a surfactant (JP-A-01-256556) or a method for producing an aqueous resin composition employing an emulsifying machine having a special finely pulverizing mechanism (JP-A-11-269206) has been proposed. However, in the method disclosed in JP-A-01-256556, the dispersed particle sizes of the emulsion are as large as from 0.5 to 7 μm, whereby there has been a problem from the viewpoint of the dispersion stability. Further, in the method disclosed in JP-A-11-269206, a special emulsifying machine is employed, where the cost will increase, and practically, a surfactant or a basic substance is essentially required, and there has been a problem that the particle size of the aqueous dispersion is limited to a level of about 0.15 μm, and the blending property is poor when blended with an emulsion having a finner particle size.
As a method to make the dispersed particle size to be fine, a method has also been proposed wherein a modified polyolefin is dissolved in an ether solvent such as propylene glycol monopropyl ether, and water is dropwise added and dispersed in the presence of a base, whereupon the solvent is distilled off (JP-A-2004-018659 (US2005-143527)). However, such an ether solvent is compatible infinitely with water at a commonly employed temperature of from room temperature to the boiling point, whereby water and the ether can not be separated by liquid-liquid separation after the distillation. Accordingly, the distilled solvent is required to be entirely recovered by distillation, thus leading to problems from the environmental and process aspects. Or, a highly hydrophobic polyolefin has a low solubility in an ether solvent and is hardly soluble unless it is in a state having polar groups added to the polyolefin to some extent. Therefore, there is a problem such that an ether solvent can not be used as a solvent for a polymerization reaction or a modifying reaction of a polyolefin.